Which ancient astronomer listed 48 constellations in the Almagest? The answer is Ptolemy.
Claudius Ptolemy was a Greco-Roman astronomer who worked in Alexandria during the second century CE. His Almagest, written in Greek, presented mathematical models of the heavens and included a star catalogue containing 48 constellations. The catalogue recorded the positions and brightnesses of more than 1,000 stars by ancient astronomical standards.
Ptolemy did not invent every constellation in the catalogue. Much of the tradition came from earlier Greek astronomy, especially the work associated with Hipparchus and earlier sky descriptions. However, the Almagest preserved and organized the material in a form that strongly influenced astronomy for centuries. That is why Ptolemy is the canonical answer to this question.
The 48 ancient constellations became the foundation for many of the modern IAU constellations. They do not match the modern list exactly: southern-sky constellations were added later, and Ptolemy’s Argo Navis was eventually divided into Carina, Puppis and Vela. Hipparchus is a plausible distractor because his earlier star catalogue influenced Ptolemy’s work, but he did not author the Almagest.